Hi Chris,
Yes, it is possible. You need to implement a scripting action taking an input of type string and returning an Array/VC:DistributedVirtualPortgroup (assuming that is the type of portgroups you want to select in the presentation). Then you need to implement the action code that will compute the list of valid portgroups for the customer name the action gets as an input.
Then, in the second input field presentation you need to add a property "Predefined list of elements" and bind it to a call to your scripting action. Property's value should look something like:
GetAction("com.my.actions","getPortgroupsPerCustomer").call( #customer )
where:
#customer - references the first presentation field where you select the customer
getPortgroupsPerCustomer - the name of your scripting action
com.my.actions - the sample category name of your scripting action